1. Field of the Invention
This invention relates to a fixing device and an image forming apparatus equipped with the fixing device.
2. Description of the Related Art
As a fixing device used in various types of image forming apparatuses, such as a copier, a printer, a facsimile machine, a multifunction device, etc., that which employs a thin fixing belt consisting of a metal substrate and an elastic rubber layer or the like is well known. Such a thin fixing belt decreases heat capacity, and accordingly the amount of energy needed to heat the fixing belt is significantly reduced thereby shortening both a warm-up time period (e.g. reload temperature) when a power is turn on or the like and a time to first print (i.e., a time period after receiving a print request until sheet ejection through preparation and conducting printing).
Conventionally, as disclosed in Japanese Patent Application Publication No. 2007-334205 (JP-2007-334205-A), a fixing device employs an endless belt (i.e., a fixing belt), a pipe-shaped metal heat conductor disposed inside the loop formed by the endless belt, a heat source in the metal heat conductor, and a pressing roller contacting the metal heat conductor via the endless belt forming a nip N thereon. The endless belt is rotated as the pressing roller rotates and is guided by the metal heat conductor. The endless belt is wholly warmed up by a heat source via the metal heat conductor. This can reduce a time to first print.
To further save energy and reduce the first print time, Japanese Patent Application Publication No. 2007-233011 (JP-2007-233011-A) proposes a fixing device that directly heats an endless belt not via the metal heat conductor.
As shown in FIG. 14, such a fixing device omits the above-described pipe-shaped metal heat conductor from inside the endless belt 100, and is provided with a plate-state nip formation unit 500 facing the pressing roller 400 instead. Consequently, since the endless belt 100 is directly heated by the heat source 300 at a position other than the nip formation unit 500, heat transfer efficiency is significantly improved saving power. Because of this, the first print time is further decreased while saving the cost due to exclusion of the metal heat conductor.
However, based on recent environmental requirements, further energy saving is expected of the fixing device. For example, in a conventional image forming apparatus, when a print job does not exist, i.e., during a standby state, to instantly start printing upon receiving a print job, an fixing roller (or a fixing belt) and a pressing roller in a fixing device is controlled to maintain a prescribed temperature by repeatedly turning on/off a heater located in the fixing device e, which is not an energy-efficient operation.
Conventional attempts to reduce power consumption can be roughly classified into two types: one that improves the above-described heating system as disclosed in JP-2007-334205-A and JP-2007-233011-A, and another that improves controlling an internal heater as disclosed, for example, in Japanese Patent No. 4423070 (JP-4423070-B). However, there is a limit to how much power can be saved with these approaches.
Further, a fixing device capable of saving power by reducing radiation of heat therefrom has been proposed as disclosed in Japanese Patent Application Publication No. 2006-133318 (JP-2006-133318-A). However, a structure of the fixing unit 20 has a complex structure.